1. Field of the Invention
The present invention relates to a process for the consolidation or stabilization of radioactive wastes in a glass matrix, wherein a glass smelt which has been enriched with the active material is solidified through cooling.
Glass serves as an extensively chemically and thermally resistant material and the processes for the vitrification of highly-radioactive wastes from reprocessing installations (for nuclear fuel) evidence a universally high degree of development. The fission product solutions are concentrated, mixed with glass formers or glass frit, dried, calcinated and melted into glass either in charges or in continuous operation in furnaces, and filled into final repository containers. These are slowly cooled in order to avoid fissures and stresses in the glass, and finally brought to the final repository location.
2. Discussion of the Prior Art
In the selection of the glass composition one is forced to undertake a certain compromise since highly-resistant glasses, which contain up to 80% SiO.sub.2, require temperatures of 1300.degree. to 1600.degree. C. for smelting. At these high temperatures, considerable proportions of the radioactive material are volatilized. Consequently, the actually employed glasses contain a lower SiO.sub.2 content besides oxides of Li, Na, K, Mg, Ca, Ba, B, Ti and similar additives which are known from the glass technology. However, such glasses have evidenced themselves as not being absolutely leach-resistant, particularly when they are subjected to conditions which are currently employed for leaching tests. Thus, fission product-containing boron silicate glass already contains after a 500 hour long exposure to carnallite leach at 200.degree. C. and 100 at, heavily deposited crusts of corroded glass.
Investigated already as more extensively leach-resistant inclusion compounds have been aluminum oxide-containing glasses or ceramic compounds, such as are indicated in the summarizing report by G. Sachse and H. Rosenberger in "Kernenergie" 10 (1967), pages 205-210. Mentioned herein as particularly leach-resistant glass systems, among others are vitreous smelts based on Al.sub.2 O.sub.3, CaO, Na.sub.2 O, B.sub.2 O.sub.3 and SiO.sub.2. Such aluminum-containing boron silicate glasses require temperatures at or above 1500.degree. C. for smelting, which are undesirably high for the fission product stabilization.
Furthermore, a considerable tendency towards spontaneous crystallization consists in glasses engendering physical and chemical changes which can extensively exert themselves on the mechanical destructability, leaching resistance and heat conductivity, as well as on other properties. Thus, it has already attempted to convert such glasses into glass ceramics having further improved properties through controlled crystallizations (A.De, et al in "Atomwirtschaft" 1975, pages 359-360). For the formation of such a glass ceramic, the glass compound which has already been smelted at a high temperature must be subjected to an up to 24-hour long controlled thermal treatment at high temperatures in the proximity of the smelting point. In a larger scale, such techniques have been ascertained as being realizable only with difficulty and as not quite satisfactory.
Due to this reason, there has been developed in Sweden the socalled Asea process for fission product stabilization in which calcinated fission products are mixed with aluminum oxide and are solidified into a monolith under a compressive pressure of a few 100 atm at about 800.degree. to 900.degree. C., which should be stable in carnallite leach. Such a monolith formation under extraordinarily high pressures hardly appears to be a standard process for the stabilization of radioactive wastes.
This means, that in order to achieve the best possible leaching resistance from glass-like or ceramic-like fission product-containing compounds, there are employed either extremely complex or not fully investigated techniques or relatively high smelting temperatures, so as to cause fear of activity losses.